In To Kill a Mockingbird, what does the reader learn in chapter 3 about the Ewells?
It is in this chapter that Atticus tells Jem and Scout (and us) about the background of the Ewells and why Burris Ewell is able to get away with playing truant whilst Scout is unable to do so. Note that Atticus tells that they have been a "disgrace to Maycomb for three generations," which is how they are allowed to get away with things that other people are not allowed to get away with:
Atticus said that the Ewells had been the disgrace of Maycomb for three generations. None of them had done an honest day's work in his recollection... They were people, but they lived like animals...
He said that the Ewells were members of an exclusive society made up of Ewells. In certain circumstances the common folk judiciously allowed them certain privileges by the simple method of becoming blind to some of the Ewells' activities. They didn't have to go to school, for one thing. Another thing, Mr. Bob. Ewell, Burris's father, was permitted to hunt and trap out of season.
The reason for this difference in treatment is that Mr. Ewell spends his relief checks on alcohol and his children are often left to cry in their hunger. That is why people turn a blind eye to Mr. Ewell's illegal activities, even when they infringe the law. Giving him a chance to feed his family is more important than upholding a law in this case. Thus this chapter reveals very important information regarding this family, the way they are treated by the residents of Maycomb and also the desperate position that they find themselves in.